Method and device for comprehensive anti-tachyarrhythmia therapy

ABSTRACT

A method and apparatus for delivering therapy to treat ventricular tachyarrhythmias is described. In one embodiment, neural stimulation, anti-tachycardia pacing, and shock therapy are employed in a progressive sequence upon detection of a ventricular tachycardia.

FIELD OF THE INVENTION

This invention pertains to methods and system for treating cardiacarrhythmias with electrical stimulation.

BACKGROUND

Tachyarrhythmias are abnormal heart rhythms characterized by a rapidheart rate. Examples of tachyarrhythmias include supraventriculartachycardias (SVT's) such as atrial tachycardia (AT), and atrialfibrillation (AF). The most dangerous tachyarrhythmias, however, areventricular tachyarrhythmias: ventricular tachycardia (VT) andventricular fibrillation (VF). Ventricular rhythms occur when re-entryof a depolarizing wavefront in areas of the ventricular myocardium withdifferent conduction characteristics becomes self-sustaining or when anexcitatory focus in the ventricle usurps control of the heart rate fromthe sinoatrial node. The result is rapid and ineffective contraction ofthe ventricles out of electromechanical synchrony with the atria. Mostventricular rhythms exhibit an abnormal QRS complex in anelectrocardiogram because they do not use the normal ventricularconduction system, the depolarization spreading instead from theexcitatory focus or point of re-entry directly into the myocardium.Ventricular tachycardia is typically characterized by distorted QRScomplexes that occur at a rapid rate, while ventricular fibrillation isdiagnosed when the ventricle depolarizes in a chaotic fashion with noidentifiable QRS complexes. Both ventricular tachycardia and ventricularfibrillation are hemodynamically compromising, and both can belife-threatening. Ventricular fibrillation, however, causes circulatoryarrest within seconds and is the most common cause of sudden cardiacdeath.

Cardioversion (an electrical shock delivered to the heart synchronouslywith the QRS complex to terminate VT or AF) and defibrillation (anelectrical shock delivered without synchronization to the QRS complex toterminate VF) can be used to terminate most tachyarrhythmias. Theelectric shock terminates the tachyarrhythmia by depolarizing all of themyocardium simultaneously and rendering it refractory. A class ofcardiac rhythm management devices known as an implantable cardioverterdefibrillator (ICD) provides this kind of therapy by delivering a shockpulse to the heart when the device detects tachyarrhythmias.

Another type of electrical therapy for tachycardia is anti-tachycardiapacing (ATP). In ventricular ATP, the ventricles are competitively pacedwith one or more pacing pulses in an effort to interrupt the reentrantcircuit causing the tachycardia. Modern ICD's typically have ATPcapability so that ATP therapy is delivered when VT is detected, while ashock pulse can be delivered to terminate both VT and VF. Althoughcardioversion/defibrillation will terminate ventricular tachycardia, itconsumes a large amount of stored power from the battery and results inpatient discomfort owing to the high voltage of the shock pulses. It isdesirable, therefore, for the ICD to terminate a tachyarrhythmiawhenever possible without using shock therapy. Devices have thereforebeen programmed to use cardioversion/defibrillation shocks to terminatefibrillation and certain high rate tachycardias and to use ATP to treatlower rate tachycardias.

SUMMARY

Described herein is a device and method for treating atrial ortachyarrhythmias which, in addition to ATP and shock therapy, employsneural stimulation. The neural stimulation may be parasympatheticstimulation or sympathetic inhibition. In an exemplary embodiment, adevice is programmed to deliver the therapies in a tiered manner inresponse to detection of a tachyarrhythmia. The neural stimulation aidsin terminating certain tachyarrhythmias without resort to shock therapyand can also be used in conjunction with shock therapy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the physical placement of an implantable cardiacdevice.

FIG. 2 is a block diagram of a cardiac rhythm management device withATP, neuro-modulation and cardioversion/defibrillation capability.

FIG. 3 is a flow diagram showing the therapy delivery steps performed ina particular embodiment.

FIG. 4 is a flow diagram showing the configuration steps performed in aparticular embodiment.

DETAILED DESCRIPTION

Cardiac rate, contractility, and excitability are known to be modulatedby centrally mediated reflex pathways. Baroreceptors and chemoreceptorsin the heart, great vessels, and lungs, transmit cardiac activitythrough vagal and sympathetic afferent fibers to the central nervoussystem. Activation of sympathetic afferents triggers reflex sympatheticactivation, parasympathetic inhibition, vasoconstriction, andtachycardia. In contrast, parasympathetic activation results inbradycardia, vasodilation, and inhibition of vasopressin release. Thepresent disclosure relates to an implantable device with the capabilityof sensing the presence of a tachyarrhythmia and terminating thearrhythmia with a combination of neural stimulation, ATP, andhigh-voltage shocks. Neural stimulation, in the form of sympatheticinhibition or parasympathetic activation, decreases myocardialexcitability and conduction time to thereby increase the likelihood ofarrhythmia termination. The neural stimulation thus improves theefficacy of ATP, and/or lowers defibrillation threshold for shocktherapy. Described below is an exemplary device which may be configuredto treat atrial and ventricular tachyarrhythmias with ATP, shocktherapy, and neural stimulation. A particular implementation algorithmis also described.

1. Hardware Platform

As shown in FIG. 1, an implantable cardiac device 100 for deliveringtachyarrhythmia therapy is typically placed subcutaneously orsubmuscularly in a patient's chest with leads 200 threaded intravenouslyinto the heart to connect the device to electrodes 300 used for sensingand pacing of the atria and/or ventricles. Electrodes may also bepositioned on the epicardium by various means. A programmable electroniccontroller causes electrical stimulation for terminating atachyarrhythmia to be output in response to sensed cardiac electricalactivity. The device senses intrinsic cardiac electrical activitythrough a sensing channel which incorporates internal electrodesdisposed near the chamber to be sensed. A depolarization wave associatedwith an intrinsic contraction of the atria or ventricles that isdetected by the device is referred to as an atrial sense or ventricularsense, respectively, and the atrial and ventricular rates may bedetermined by measuring the intervals between senses. Also shown in thefigure is a neural stimulation electrode 110 which may be a direct nervecuff or a transvascular lead placed in, for example, the internaljugular vein for transvascular stimulation of the vagus nerve. The 110electrode communicates with the device 100 via a lead 105 which passessubcutaneously from the device 100 to a point of venous access in theupper chest or neck.

FIG. 2 is a system diagram of a microprocessor-based cardiac rhythmmanagement device with the capability of deliveringcardioversion/defibrillation shocks, anti-tachycardia pacing therapy toeither the ventricles or the atria, and neural stimulation. Thecontroller of the device is a microprocessor 10 which communicates witha memory 12 via a bidirectional data bus. The controller could beimplemented by other types of logic circuitry (e.g., discrete componentsor programmable logic arrays) using a state machine type of design, buta microprocessor-based system is preferable. As used herein, the term“circuitry” should be taken to refer to either discrete logic circuitryor to the programming of a microprocessor. Shown in the figure are threeexemplary sensing and pacing channels designated “a” through “c”comprising bipolar leads with ring electrodes 33 a-c and tip electrodes34 a-c, sensing amplifiers 31 a-c, pulse generators 32 a-c, and channelinterfaces 30 a-c. Each channel thus includes a pacing channel made upof the pulse generator connected to the electrode and a sensing channelmade up of the sense amplifier connected to the electrode. The channelinterfaces 30 a-c communicate bidirectionally with microprocessor 10,and each interface may include analog-to-digital converters fordigitizing sensing signal inputs from the sensing amplifiers andregisters that can be written to by the microprocessor in order tooutput pacing pulses, change the pacing pulse amplitude, and adjust thegain and threshold values for the sensing amplifiers. The sensingcircuitry of the pacemaker detects a chamber sense, either an atrialsense or ventricular sense, when an electrogram signal (i.e., a voltagesensed by an electrode representing cardiac electrical activity)generated by a particular channel exceeds a specified detectionthreshold. Pacing algorithms used in particular pacing modes employ suchsenses to trigger or inhibit pacing. The intrinsic atrial and/orventricular rates can be measured by measuring the time intervalsbetween atrial and ventricular senses, respectively, and used to detectatrial and ventricular tachyarrhythmias.

The electrodes of each bipolar lead are connected via conductors withinthe lead to a MOS switching network 70 controlled by the microprocessor.The switching network is used to switch the electrodes to the input of asense amplifier in order to detect intrinsic cardiac activity and to theoutput of a pulse generator in order to deliver a pacing pulse. Theswitching network also enables the device to sense or pace either in abipolar mode using both the ring and tip electrodes of a lead or in aunipolar mode using only one of the electrodes of the lead with thedevice housing (can) 80 or an electrode on another lead serving as aground electrode. A shock pulse generator 60 is also interfaced to thecontroller for delivering a defibrillation shock via a pair of shockelectrodes 61 to the atria or ventricles upon detection of a shockabletachyarrhythmia.

The controller may be programmed with a plurality of selectable ATPpacing protocols that define the manner in which anti-tachycardia pacingis delivered. In a microprocessor-based device, the output of pacingpulses is controlled by a pacing routine that implements the selectedpacing protocol as defined by various parameters. A data structurestored in memory contains the parameter sets that define each of theavailable pacing protocols. Different protocols are apt to be moresuccessful than others in terminating particular tachyarrhythmias thatmay differ as to rate and/or depolarization pattern. For this reason,modern cardiac rhythm management devices are capable of employing anumber of different ATP protocols to deliver therapy.

Neural stimulation channels are incorporated into the device fordelivering parasympathetic stimulation and/or sympathetic inhibition,where one channel includes a bipolar lead with a ring electrode 43 and atip electrode 44, a pulse generator 42, and a channel interface 40, andthe other channel includes a bipolar lead with a ring electrode 53 and atip electrode 54, a pulse generator 52, and a channel interface 50.Other embodiments may use unipolar leads in which case the neuralstimulation pulses are referenced to the can or another electrode. Thepulse generator for each channel outputs a train of neural stimulationpulses which may be varied by the controller as to amplitude, frequency,and duty-cycle. In this embodiment, each of the neural stimulationchannels uses a lead which can be intravascularly disposed near anappropriate stimulation site, e.g., near a baroreceptor in the case of asympathetic inhibition channel or near a parasympathetic nerve in thecase of a parasympathetic stimulation channel. Other types of leadsand/or electrodes may also be employed. A nerve cuff electrode may beused in place of an intravascularly disposed electrode to provide neuralstimulation, where the electrode may be placed, for example, around thecervical vagus nerve bundle to provide parasympathetic stimulation oraround the aortic or carotid sinus nerve to provide sympatheticinhibition. In another embodiment, the leads of the neural stimulationelectrodes are replaced by wireless links, and the electrodes forproviding parasympathetic stimulation and/or sympathetic inhibition areincorporated into satellite units.

2. Exemplary Implementation

In an example embodiment, an implantable cardiac device with thecapability of delivering ATP therapy, shock therapy, and neuralstimulation such as illustrated by FIG. 2 is programmed to deliver thetherapies in a tiered manner in response to detection of atachyarrhythmia. The neural stimulation lead may be, for example, anexpandable stimulation lead placed in the pulmonary artery in theproximity of a high concentration of baroreceptors, a transvascular leadplaced proximal to one of the cardiac fat pads, an epicardial leadplaced in the cardiac fat pad, or a cuff electrode placed around theaortic, carotid, or vagus nerve, or a transvascular lead placed proximalto one of these nerves. The device senses the presence of atachyarrhythmia using conventional algorithms, and responds with aseries of progressively more aggressive therapies. Such comprehensiveanti-tachyarrhythmia therapy may be applied to atrial arrhythmias,ventricular arrhythmias, or both.

The device delivers anti-tachyarrhythmia therapy (i.e., ATP therapy,neural stimulation, and/or a defibrillation shock) under programmedcontrol of the microprocessor in response to sensed activity from thesensing channels. A sensing routine analyzes the electrical activityreceived from the sensing channels in order to detect tachyarrhythmias.The ventricular rate is determined by measuring the time intervalsbetween successive ventricular senses, referred to as RR intervals. Aventricular tachyarrhythmia is detected if the measured ventricular rateis above the tachycardia detection rate (TDR). Once a tachyarrhythmia isdetected, the rhythm is classified as either tachycardia or fibrillationby comparing the heart rate to the fibrillation detection rate (FDR).Example values for the TDR and FDR would be 150 bpm and 200 bpm,respectively. If the ventricular rate is at or above the FDR, alife-threatening situation exists, and the device should be programmedto immediately deliver its most aggressive therapy. Thus, in the presentembodiment, the device may be programmed to deliver shock therapy or acombination of shock therapy and neural stimulation without delay fortachyarrhythmias classified as fibrillation. For tachyarrhythmias belowthe FDR, however, the device may be programmed to deliver its differenttherapies in a progressive manner in an effort to avoid unnecessarilyaggressive intervention. The device allows a physician to program one ormore tiers for delivering the therapies, where each tier is programmedto contain one or more of the available therapies. In response to atachyarrhythmia above the TDR but below the FDR, the therapies are thendelivered in the tiered sequence until one is successful.

In one embodiment, for example, the therapies are delivered in thefollowing five-tiered sequence, where the sequence is stopped wheneverthe tachyarrhythmia is terminated:

1. Neural stimulation

2. Anti-tachycardia pacing

3. Neural stimulation+ATP

4. Shock therapy

5. Neural stimulation+shock therapy

FIG. 3 is a flow diagram showing the steps performed by a cardiac rhythmmanagement device in a particular algorithm for delivering neuralstimulation, ATP therapy, and/or shock therapy to terminate atachyarrhythmia using the approach described above. At step S1, thedevice measures RR intervals and stores the current heart rate asHR_(current). The device determines whether a tachyarrhythmia is presentby comparing the current heart rate with the TDR at step S2. If atachyarrhythmia is detected (by, e.g., determining that 8 out of 10previous RR intervals were fast or less than 60,000/TDR, where the RRintervals and the FDR are expressed in msecs and bpm, respectively) andthe current heart rate is above the FDR, the device proceeds to delivershock therapy at step S5. If the tachyarrhythmia is still present asdetermined at step S6, the device next delivers shock therapy withneural stimulation at step S7 and returns to step S1. If the currentheart rate is less than the FDR as determined at step S3, thetachyarrhythmia can be classified as a VT, and device starts a therapysequence designated as steps S4 a through S4 h. At step S4 a, neuralstimulation is delivered. If steps S4 b and S4 c determine that the VTis still present and the heart rate is still below the FDR, ATP therapyis delivered at step S4 d. If steps S4 e and S4 f determine that the VThas not been terminated and the heart rate is still below the FDR, ATPtherapy with neural stimulation is delivered at step S4 g. Step S4 hthen determines whether the VT has been terminated. If so, the devicereturns to step S1. If the VT is still present, the device proceeds withsteps S5 through S7 as necessary to try to terminate the VT with shocktherapy or shock therapy with neural stimulation. The device alsoproceeds with steps S5 through S7 if the heart rate is found to at orabove the FDR at steps S4 c and S4 f during the therapy sequence.

The embodiment described with reference to FIG. 3 dealt with treatingventricular arrhythmias. A similar algorithm employing the sameprogressive therapy sequence could be executed by the device to treatatrial tachyarrhythmias. In that case, an atrial tachyarrhythmia wouldbe detected by measuring the intervals between atrial senses, the TDRand FDR would be referenced to atrial rates, the ATP therapy would beatrial ATP, and the shock therapy would be cardioversion shocks designedto terminate AF. When a device is configured for delivering ventricularanti-tachycardia pacing or shocks in order to terminate ventriculartachycardias and for delivering atrial anti-tachycardia pacing or atrialconversion shocks in order to terminate atrial tachyarrhythmias, and itis important for the device to be able to distinguish between an SVT,which is an atrial tachyarrhythmia, and VT, which is a ventriculartachyarrhythmia. One way by which a device may distinguish between anSVT and VT is to employ a rate based test so that if a ventricular rateis detected that is high enough to constitute a VT, the arrhythmia iscategorically classified as a VT if the ventricular rate is also greaterthan the atrial rate. A tachycardia in which the atrial rate is greaterthan or equal to the ventricular rate, on the other hand, may be a VT,an SVT, or a dual tachycardia where both VT and SVT are presentsimultaneously. Since VT is the more serious condition and requiresprompt treatment, the rate based test may require that one or moreadditional rate based criteria be met before VT is ruled out. Suchcriteria, which help identify SVT, may include instability of theventricular rate, lack of sudden onset, and an atrial rate above aspecified atrial tachyarrhythmia threshold. Only if these one or moreadditional criteria are met is the tachycardia then classified as anSVT. The atrial tachyarrhythmia threshold may also be used to furtherclassify the SVT as an atrial tachyarrhythmia requiring treatment, suchas atrial fibrillation, or as ST which is not considered an arrhythmiaand requires no treatment. In order to more accurately detect an SVT, acombined morphology/rate based test may be used instead of the ratebased test described above for SVT/VT discrimination. In a combinedmorphology/rate based test, a morphology criterion based upon amorphology analysis of electrogram waveforms is used to provide anadditional criterion for detecting an SVT. For example, SVT may bedetected if: 1) the atrial rate is greater than or equal to theventricular rate and, 2) some minimum number of ventricular beats arejudged as normally conducted as determined by a morphology criterioninvolving morphology analyses of electrogram waveforms. An exemplaryimplementation of a morphology criterion for SVT/VT discrimination isdisclosed in U.S. Pat. No. 6,449,503, assigned to Cardiac Pacemakers,Inc., the disclosure of which is hereby incorporated by reference in itsentirety.

In one particular embodiment, the device is programmable with respect tothe TDR, FDR, number of therapy tiers, and the particular therapy ortherapies included within each tier. The device accepts user inputs viatelemetry interface 85 (FIG. 2) to define a specified tachycardiadetection rate (TDR) and a fibrillation detection rate (FDR), to definea number of TDR therapy tiers and select one or more therapies forinclusion in each TDR tier, to define a number of FDR therapy tiers andselect one or more therapies for inclusion in each FDR tier. The one ormore therapies which may be included in a TDR or FDR tier include neuralstimulation, anti-tachycardia pacing (ATP) therapy, and shock therapy aswell as possibly other types of tachyarrhythmia therapy such as drugdelivery. Upon detection of a tachyarrhythmia, the device then deliversthe therapies of the defined FDR tiers in a therapy sequence if theheart rate is above the FDR where the therapy sequence is stopped if thetachyarrhythmia is terminated. If the tachyarrhythmia is below the FDR,the device delivers the therapies of the defined TDR tiers in a therapysequence wherein the therapy sequence is stopped if the tachyarrhythmiais terminated. FIG. 4 illustrates the steps a user would take inprogramming the device to treat tachyarrhythmias with comprehensivetherapy. Such settings may be separately programmable for atrial andventricular tachyarrhythmias. At step A1, the user defines the TDR andFDR. At step A2, the number of FDR therapy tiers used to treattachyarrhythmias at or above the FDR are defined. At step A3, theparticular therapy or therapies which are to be included in the definedFDR therapy tiers are selected. The number of TDR therapy tiers used totreat tachyarrhythmias between the TDR and the FDR are defined at stepA4, and the therapy or therapies included in the each TDR tier isselected at step A5.

In another embodiment, the device may also be programmed such that thetherapy sequence delivered upon detection of a tachyarrhythmia varieswith the detected heart rate. For example, a plurality of heart rateranges between the TDR and the FDR may be defined with a differenttherapy sequence for each such heart rate range. The number of heartrate ranges and the boundaries defining the ranges may be eitherprogrammable or fixed with the option of enabling the feature or not.For example, two heart rate ranges may be defined between the TDR andthe FDR so that a VT is designated as either VT or FVT (fast VT).Separate therapy sequences could then be programmed for each type of VT.For example, the therapy sequence as described above starting withneural modulation alone could be used for VT, while the sequence wouldstart with neural modulation together with ATP for FVT.

In another embodiment, device may be programmed to automatically alterthe therapy sequence used to treat tachyarrhythmias in accordance with arecorded success/failure ratio for each therapy. In an exemplaryimplementation, the device records the successes and failures of eachtherapy in terminating tachyarrhythmias for a plurality of heart rateranges between the TDR and the FDR and computes a success/failure ratiofor each therapy at each heart rate range. If the success/failure ratiofor a particular therapy at a particular heart rate range drops below aspecified threshold value, or drops below a specified thresholddifference from that of another therapy in the therapy sequence, thenthe unsuccessful therapy is discontinued from the therapy sequence forthe particular heart rate range.

Although the invention has been described in conjunction with theforegoing specific embodiments, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

1. A method for delivering anti-tachyarrhythmia therapy, comprising:generating an electrogram signal representing electrical activity in acardiac chamber; detecting chamber senses and measuring a time intervalbetween such senses to determine a current heart rate; detecting atachyarrhythmia when the heart rate exceeds a specified tachycardiadetection rate (TDR); upon detection of a tachyarrhythmia, deliveringshock therapy if the heart rate is above a fibrillation detection rate(FDR); upon detection of a tachyarrhythmia below the FDR, deliveringtherapies in the following five-tiered sequence, where the sequence isstopped whenever the tachyarrhythmia is terminated: 1) neuralstimulation, 2) anti-tachycardia pacing (ATP), 3) neural stimulation andATP, 4) shock therapy, and 5) neural stimulation and shock therapy;recording the successes and failures of each therapy in terminatingtachyarrhythmias for a plurality of heart rate ranges between the TDRand the FDR and computing a success/failure ratio for each therapy ateach heart rate range; and, discontinuing a therapy from the sequencefor a particular heart rate range if the success/failure ratio for aparticular therapy at that particular heart rate range drops below aspecified threshold value.
 2. The method of claim 1 further comprisingdiscontinuing a therapy from the sequence for a particular heart raterange if the success/failure ratio for a particular therapy at thatparticular heart rate range drops below a specified threshold differencefrom that of another therapy in the therapy sequence.
 3. The method ofclaim 2 further comprising accepting an input defining the number ofheart rate ranges and the boundaries defining the ranges.
 4. The methodof claim 1 further comprising delivering neural stimulation with shocktherapy if the heart rate is above the FDR.
 5. The method of claim 1wherein the cardiac chamber is a ventricle.
 6. The method of claim 1wherein the cardiac chamber is an atrium.
 7. A cardiac device,comprising: a sensing amplifier for sensing an electrogram signalrepresenting electrical activity in a cardiac chamber; a pulse generatorfor delivering anti-tachycardia pacing (ATP); a shock pulse generatorfor delivering shock therapy; a pulse generator for delivering neuralstimulation that augments parasympathetic balance by parasympatheticstimulation or sympathetic inhibition; a controller, wherein thecontroller is programmed to: detect chamber senses and measure a timeinterval between such senses to determine a current heart rate; detect atachyarrhythmia when the heart rate exceeds a specified tachycardiadetection rate (TDR); upon detection of a tachyarrhythmia, deliver shocktherapy if the heart rate is above a fibrillation detection rate (FDR);upon detection of a tachyarrhythmia below the FDR, deliver therapies inthe following five-tiered sequence, where the sequence is stoppedwhenever the tachyarrhythmia is terminated: 1) neural stimulation, 2)anti-tachycardia pacing (ATP), 3) neural stimulation and ATP, 4) shocktherapy, and 5) neural stimulation and shock therapy; record thesuccesses and failures of each therapy in terminating tachyarrhythmiasfor a plurality of heart rate ranges between the TDR and the FDR andcompute a success/failure ratio for each therapy at each heart raterange; and, discontinue a therapy from the sequence for a particularheart rate range if the success/failure ratio for a particular therapyat that particular heart rate range drops below a specified thresholdvalue.
 8. The device of claim 7 wherein the controller is furtherprogrammed to discontinue a therapy from the sequence for a particularheart rate range if the success/failure ratio for a particular therapyat that particular heart rate range drops below a specified thresholddifference from that of another therapy in the therapy sequence.
 9. Thedevice of claim 8 wherein the controller is further programmed accept aninput defining the number of heart rate ranges and the boundariesdefining the ranges.
 10. The device of claim 7 wherein the controller isfurther programmed to deliver neural stimulation with shock therapy ifthe heart rate is above the FDR.